Communication systems, interconnecting numerous devices, such as sensors, controllers, terminals and the like, by a communication channel are used in a wide variety of applications such as process control, building energy management, reservations systems, and the like. The communication channel of such systems typically comprises a twisted pair of wires or a coaxial cable which interconnects the plurality of devices in the system. Each device on the communication channel is given an individual address so that messages can be directed from one device to another. Such messages may contain information, requests for information or commands. These messages involve various formats, but typically include the address of the destination device to which the message is being sent, the source address of the device issuing the message, a control frame indicating the type of message which is being transmitted, and data frames for containing data which is to be communicated by one device to another. Other frames, such as those used for parity checking, for indicating the start of a message and for indicating the end of a message, can be, and often are, included in the message. Each device on the communication channel, upon receipt of a message, compares the destination address contained in the message with its own address to determine whether or not it is intended to be the recipient of the message from the source device. If it is, it appropriately processes the message.
These systems have many advantages. For example, installation of such data communication systems as compared to hardwired systems is much easier. Instead of connecting each device by a separate pair of wires to each other device, only one pair of wires or a coaxial cable needs to run throughout the system. Also, such a system allows communication over long distances and can use existing data carriers such as telephone lines or satellite links in order to support data communication.
Various protocols have been developed which govern access by each device to the communication channel and to minimize the contention that would otherwise result when two or more devices attempt to transmit messages during overlapping time periods. One such protocol utilized by the present invention is a token passing protocol defined by IEEE standard 802.4. According to this protocol, any device having the token is allowed to transmit over the communication channel until either it has transmitted all of its information or a predetermined amount of time has elapsed, after which the device passes the token to the next station on the communication channel. In such a system, it is advantageous to provide in each device a list indicating the integrity of each other device in the communication system. Thus, a device need not make an attempt to communicate with a failed station.